Although it has been shown that motivation (e.g. monetary incentive) can enhance short-term memory capacity, little is known about how and where this effect occurs. Recent progress in visual short-term memory (VSTM) research suggests at least two major possibilities (Awh et al., 2006). That is, (1) motivation facilitates attentional gating at VSTM encoding, and/or (2) motivation supports VSTM maintenance by keeping sustained attention active. Previous studies, however, have failed to distinguish these two, since they manipulated motivational factors before encoding, and therefore possibly modulated the two processes simultaneously. Thus, the goal of the current study was to unravel this confound and examine the plausibility of the second account in particular. A VSTM task (Vogel & Machizawa, 2004) was combined with retro-cueing paradigm (e.g. Lepsien & Nobre, 2006). In each trial, monetary incentive cues appeared during 1,000 ms retention period (500 ms after memory array) as two pure tones that differed in frequency, which indicated high and low rewards that participants could obtain in that trial, if they answered correctly. In order to prevent random response especially in low motivation condition, we assigned negative reward (punishment) for incorrect answer in both conditions. Results showed that the VSTM performance (percentage of correct answers) was significantly facilitated in high reward condition than the other, providing the first evidence that motivation can affect VSTM maintenance directly. Possible neural bases for this effect will be discussed with the data from a follow-up ERP study.